1. Field of the Invention
The present invention relates to: a signal transmission circuit and a signal transmission method for efficiently transmitting a signal by using a signal line having a parasitic capacitance; an A/D converter for converting an analog signal into a digital signal; and a solid-state imaging element for generating an output signal corresponding to an input image.
2. Description of the Related Art
An A/D converter is used for converting an analog signal into a digital signal. A/D converters have various circuit configurations which may be used such as, for example, a sequential-comparative type, a serial-parallel type and an all-parallel type. These circuit configurations, respectively, have advantages and disadvantages with respect to conversion speed, conversion precision and the power consumption thereof.
Of the above-mentioned three types of A/D converters, the all-parallel A/D converter exhibits the best conversion speed. The all-parallel circuit configuration is a basic configuration for an A/D converter implemented as an integrated circuit. However, the all-parallel circuit configuration has several disadvantages in that the conversion precision and the power consumption thereof are not sufficiently satisfactory.
In recent years, it has become more and more necessary to reduce the power to be consumed by portable electronic equipment. As a result, an A/D converter is increasingly required to consume as low power as possible. Thus, serial-parallel A/D converters are now used more frequently than the all-parallel A/D converters.
FIG. 23 is a block diagram showing a prior art serial-parallel A/D converter. A comparator 2310 receives an analog input signal An and then compares the signal Ain with a reference voltage bank Ref. The comparator 2310 outputs upper-bit digital values DU obtained as a result of the comparison to a D/A converter (DAC) 2330 and an operator circuit 2340. The DAC 2330 outputs a lower reference voltage bank RefL. The RefL is used as a reference for determining the lower bits based on the upper-bit digital values DU output from the comparator 2310. A comparator 2320 compares the analog input signal Ain with the lower reference voltage bank RefL, thereby outputting lower-bit digital values DL to the operator circuit 2340. The operator circuit 2340 produces and outputs an A/D converted output value Dout based on the upper-bit digital values DU and the lower-bit digital values DL.
In the A/D converter shown in FIG. 23, the digital signal DU output from the comparator 2310 is converted by the DAC 2330 into an analog signal RefL. The D/A conversion performed by the DAC 2330 decreases the conversion speed and increases the power consumption. Therefore, it is desirable to realize an A/D converter without performing such a D/A conversion. In other words, it is desirable for the comparator 2310 to output an analog signal directly to the comparator 2320.
Nevertheless, if an analog signal is transmitted, then a parasitic capacitance on a signal line leads to a decrease in operation speed and an increase in power consumption. A parasitic capacitance is caused on a signal line primarily because of a multitude of switches connected to the signal line and the length of the signal line. When a circuit is required to drive a signal line having a large parasitic capacitance, the driving ability and power consumption are required to be increased disadvantageously.
Consequently, transmitting an analog signal adversely decreases the operation speed of the system and increases the power consumption thereof due to the parasitic capacitance on the signal line. Similar problems are also caused in an A/D converter for converting an analog signal.